Transcript April 29 th , 2010, GDR neutrino, Paris Livia Ludhova

Observation of
Geoneutrinos in Borexino
• Livia Ludhova
•
April 29th, 2010, GDR neutrino, Paris
(for Borexino collaboration)
Livia Ludhova for Borexino collaboration
Outline
• The Earth
– structure and composition ;
– sources of knowledge (geophysics, geology, and geochemistry);
• Geoneutrinos:
– what are they and to what questions they can answer;
• Borexino:
– experimental techniques and the detector;
• Antineutrino detection in Borexino:
– the background sources and reactor antineutrinos;
– the geoneutrino signal;
• Geoneutrino flux measurement:
– the results;
– implications and perspectives;
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Earth structure
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Earth structure
Inner Core - SOLID
• about the size of the Moon;
• Fe – Ni alloy;
• solid (high pressure ~ 330 GPa);
• temperature ~ 5700 K;
Outer Core - LIQUID
• 2260 km thick;
• FeNi alloy + 10% light elem. (S, O?);
• liquid;
•temperature ~ 4100 – 5800 K;
• geodynamo: motion of conductive
liquid within the Sun’s magnetic field;
D’’ layer: mantle –core transition
• ~200 km thick;
•seismic discontinuity;
• unclear origin;
April 29th, 2010, GDR neutrino, Paris
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Earth structure
Lower mantle (mesosphere)
• rocks: high Mg/Fe, < Si + Al;
• T: 600 – 3700 K;
• high pressure: solid, but viscose;
• “plastic” on long time scales:
CONVECTION
Transition zone (400 -650 km)
seismic discontinuity;
• mineral recrystallisation;
•: role of the latent heat?;
• partial melting: the source of midocean ridges basalts;
April 29th, 2010, GDR neutrino, Paris
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Earth structure
Upper mantle
• composition: rock type peridotite
• includes highly viscose
astenosphere on which are floating
litospheric tectonic plates
(lithosphere = more rigid upper
mantle + crust);
Crust: the uppermost part
• OCEANIC CRUST:
• created at mid-ocean ridges;
• ~ 10 km thick;
• CONTINENTAL CRUST:
• the most differentiated;
• 30 – 70 km thick;
• igneous, metamorphic, and
sedimentary rocks;
• obduction and orogenesis;
April 29th, 2010, GDR neutrino, Paris
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Seismology
P – primary, longitudinal waves
S – secondary, transverse/shear waves
April 29th, 2010, GDR neutrino, Paris
Discontinuities in the waves
propagation and the density profile
but no info about the chemical
composition of the Earth
Livia Ludhova for Borexino collaboration
Mantle-peridotite xenoliths
Geochemistry
1) Direct rock samples
* surface and bore-holes (max. 12 km);
* mantle rocks brought up by tectonics and vulcanism;
BUT: POSSIBLE ALTERATION DURING THE TRANSPORT
2) Geochemical models:
– composition of direct rock samples +
chondritic meteorites + Sun;
Bulk Silicate Earth (BSE) models:
medium composition
of the “re-mixed” crust + mantle,
i.e., primordial mantle before the crust
differentiation and after the Fe-Ni core
separation;
(original: McDonough & Sun 1995)
April 29th, 2010, GDR neutrino, Paris
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Earth heat flow
Bore-hole measurements
• Conductive heat flow from
bore-hole temperature
gradient;
• Total heat flow :
31+1 TW or 44+1 TW
(same data, different analysis)
Different assumptions concerning
the role of fluids in the zones of
mid ocean ridges.
Global Heat Flow Data (Pollack et al.)
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Sources of the Earth heat
•
Total heat flow (“measured”): 31+1 or 44+1 TW
•
Radiogenic heat flow (BSE composition) cca. 19 TW
the main long-lived radioactive elements within the Earth:
238U, 232Th,
and 40K
9 TW crust (mainly continental), 10 TW mantle, 0 TW core;
U, Th, K are refractory lithophile elements (RLE)
Volatile /Refractory:
Low/High condensation temperature
Lithophile – like to be with silicates: during partial melting they tend to stay in the
liquid part. The residuum is depleted. Accumulated in the continental crust. Less in
the oceanic crust. Mantle even smaller concentrations. Nothing in core.
•
Other heat sources (possible deficit of 44-19 = 25 TW!)
– Residual heat: gravitational contraction and extraterrestrial impacts in
the past;
– 40K in the core;
– nuclear reactor; (BOREXINO rejects a power > 3 TW at 95% C.L.)
– mantle differentiation and recrystallisation;
IMPORTANT MARGINS FOR ALL DIFFERENT MODELS OF THE EARTH STRUCTUE
Geoneutrinos: antineutrinos from the Earth
•
238U, 232Th, 40K
chains (T1/2 = (4.47, 14.0, 1.28) x 109 years, resp.):
 206Pb + 8 a + 8 e- + 6 anti-neutrinos + 51.7 MeV
232Th  208Pb + 6 a + 4 e- + 4 anti-neutrinos + 42.8 MeV
40K  40Ca + e- + 1 anti-neutrino + 1.32 MeV
238U
Earth shines in antineutrinos: flux ~ 106 cm-2 s-1
leaving freely and instantaneously the Earth interior
(to compare: solar neutrino flux ~ 1010 cm-2 s-1)
– released heat and anti-neutrinos flux in a well fixed ratio!
• Possible answers to the questions:
– What is the radiogenic contribution to the terrestrial heat??
– What is the distribution of the radiogenic elements within the Earth?
• how much in the crust and mantle
• core composition: Ni+Fe and 40K?? geo-reactor ? (Herndon 2001)
– Is the BSE model compatible with geoneutrino data?
April 29th, 2010, GDR neutrino, Paris
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Detecting geo-n: inverse b-decay
Energy threshold of
Tgeo-n = 1.8 MeV
g (0.511 MeV)
i.e. Evisible ~ 1 MeV
ne
PROMPT SIGNAL
Evisible = Te + 2*0.511 MeV =
e+
p
= Tgeo-n – 0.78 MeV
g (0.511 MeV)
p
Low reaction s 
large volume detectors
Liquid scintillators
Radioactive purity &
underground labs
n
n
neutron thermalization
up to cca. 1 m
April 29th, 2010, GDR neutrino, Paris
DELAYED SIGNAL
mean n-capture time on p
250 ms
g (2.2 MeV)
Livia Ludhova for Borexino collaboration
Geoneutrinos energy spectra
(theoretical calculations)
1.8 MeV = threshold for
inverse b-decay reaction
Geoneutrinos
energy range
Tgeo-n = 1.8 - 3.3 MeV
Evisible ~ 1 – 2.5 MeV
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Running and planned experiments
having geoneutrinos among their aims
Mantovani et al., TAUP 2007
Only 2 running experiments having a potential to measure geoneutrinos
KamLand in Kamioka, Japan
S(reactors)/S(geo) ~ 6.7
OCEANIC CRUST
April 29th, 2010, GDR neutrino, Paris
Borexino in Gran Sasso, Italy
S(reactors)/S(geo) ~ 0.3 !!! (2010)
CONTINENTAL CRUST
Livia Ludhova for Borexino collaboration
Expected geoneutrino signal at Borexino site
Slope – fixed by the reactions energetics
Intercept + width –
site dependent, U+Th distribution
Region allowed by the BSE
geochemical model
S(U+Th) [TNU]
Allowed region – consistent with
geophysical & geochemical data
Minimum from known U+Th
concentrations in the crust
Maximum given by the total
Earth heat flow
Heat (U+Th) [TW]
for LNGS Mantovani et al., TAUP 2007
1 TNU ( Terrestrial Neutrino Unit) = 1 event/ 1032 protons/year
Important local geology: cca. half of the signal comes from within 200 km range!!
April 29th, 2010, GDR neutrino, Paris
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Abruzzo
120 Km from Rome
Laboratori
Nazionali del
Gran Sasso
External Laboratories
Assergi (AQ)
Italy
~3500 m.w.e
Borexino detector + fluid plants
Underground labs
Torino – 12 dicembre 2007
M. Pallavicini - Università di Genova & INFN
Borexino Detector
Scintillator:
270 t PC+PPO (1.5 g/l)
in a 150 mm thick
inner nylon vessel (R = 4.25 m)
Stainless Steel Sphere:
R = 6.75 m
2212 PMTs
1350 m3
Buffer region:
PC+DMP quencher (5 g/l)
4.25 m < R < 6.75 m
Water Tank:
g and n shield
m water Č detector
208 PMTs in water
2100 m3
Outer nylon vessel:
R = 5.50 m
(222Rn barrier)
Carbon steel plates
the smallest radioactive background in the world:
9-10 orders of magnitude smaller
than the every-day environment
20 steel legs
Data acquisition and data structure
Charged particles and g produce scintillation light: photons hit inner PMTs;
DAQ trigger: > 25 inner PMTs (from 2212) are hit within 60-95 ns:

16 ms DAQ gate is opened;

Time and charge of each hit detected;

Each trigger has its GPS time;
“cluster” of hits = real physical event
Outer detector gives a muon veto if at least 6 outer PMTs (from 208) fire;
Calibration
With a,b,g and neutron sources
in 300 positions on and off axis
Insertion
Am-Be source
Comparison Monte Carlo (G4BX) - data
Source inside
Borexino
Energy resolution
10% @ 200 keV
8% @ 400 keV
6% @ 1 MeV
Spatial resolution
35 cm @ 200 keV
16 cm @ 500 keV
Event selection
An anti-neutrino candidate is selected
using the following cuts
AmBe calibration
prompt
1) Light yield of prompt signal > 410 p.e.
2) Light yield of delayed signal:
delayed
700p.e. ≤ Qdelayed ≤ 1250p.e.
3) Correlated time: 2 ms ≤ Dt ≤ 1280 ms
4) Correlated distance: DR < 1m
Selected events can be due to:
5) Reconstructed vertex of prompt signal:
RInnerVessel – Rprompt ≥ 25 cm
• geoneutrinos
Total detection efficiency determined by
MC simulations: 0.85 ± 0.01
•background ;
April 29th, 2010, GDR neutrino, Paris
•reactor antineutrinos
Livia Ludhova for Borexino collaboration
Reactors
Survival probability vs distance
∆m212 = 7.65 ·10− 5 eV2
sin2θ12=0.304
CHOOZ
KamLAND
Proposal
BOREXINO Lmean ~ 1000 km
194 reactors
245 world non European reactors: ~2% contribution
April 29th, 2010, GDR neutrino, Paris
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Calculation of reactor anti-n signal
From the literature:
Ei : energy release per fission of isotope i (Huber-Schwetz 2004);
Φi: antineutrino flux per fission of isotope i (polynomial parametrisation, H-Sch‘04);
Pee: oscillation survival probability;
Calculated:
Tm: live time during the month m;
Lr: reactor r – Borexino distance;
235U
239Pu
238U
241Pu
Data from nuclear agencies:
Prm: thermal power of reactor r in month m (IAEA , EDF, and UN data base);
fri: cpower fraction of isotope i in reactor r;
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Expected signal and its error
Fn (En>1.8 MeV)= (9.0 +0.5)104 cm-2s-1
(5.7+0.3) events/yr/100 t
s~10-44 cm2 Nprotons = 6x1030 in 100 tons
Source of error
Error (%)
Oscillations: Δm2
Oscillations: ϑ12
Energy per fission of isotope i:
Ei
Flux shape: Φi(Eν)
Cross section: σ(E)
Thermal power: Prm
Long lived isotopes in spent fuel
±0.02%
±2.6%
Fuel composition: fri
±3.2%
Reactor – Borexino distance Lr
±0.4%
TOTAL
±0.6%
±2.5%
±0.4%
±2%
±1%
±5.38%
April 29th, 2010, GDR neutrino, Paris
Energy spectrum of prompt events
235U
239Pu
238U
241Pu
Sum with oscil.
Sum NO oscil.
Prompt energy (MeV)
Livia Ludhova for Borexino collaboration
Background sources
μ μ
Reactions which can mimick the
golden coincidence:
1) Cosmogenic muon induced:
•9Li e 8He decaying b-n;
(2 s time cut after each internal m)
•Neutrons of high energies;
(2 ms time cut after each external m)
•Non-identified muons;
μ
μ
Limestone rock
n
n
n,
9Li,8He
n
2) Accidental coincidences;
3) Due to the internal radioactivity:
(a,n) and (g,n) reactions
April 29th, 2010, GDR neutrino, Paris
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Summary of backgrounds
Background source
events/(100 ton-year)
Cosmogenic 9Li and 8He
0.03 ± 0.02
Fast neutrons from μ in Water Tank (measured)
< 0.01
Fast neutrons from μ in rock (MC)
< 0.04
Non-identified muons
0.011 ± 0.001
Accidental coincidences
0.080 ± 0.001
Time correlated background
< 0.026
(γ,n) reactions
< 0.003
Spontaneous fission in PMTs
0.003 ± 0.0003
(α,n) reactions in the scintillator [210Po]
0.014 ± 0.001
(α,n) reactions in the buffer [210Po]
< 0.061
0.14 ± 0.02
TOTAL
Aspettiamo: 2.5 geo-n/(100ton-year)
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Shape of the expected spectra
Theoretical spectra: input to MC
MC output:
includes detector response function
Sum NON oscillation
Geo-ν
reactors
Geo-ν
reactors
USED IN THE UNBINNED
MAXIMUM LIKELIHOOD
FIT OF THE DATA
Results: 21 candidates selected
in 483 live days (252.6 ton-year after all cuts)
Radial distribution
Events vs time
Unbinned max. likelihood fit of data
Realative time distance
tBest-fit = 279 ms
Statistical significance of the result
G. Bellini et al., PLB 687 (2010) 299-304.
Signal evidence
at 4.2s
68.3 % c.l. 99.7% c.l.
KamLand
Borexino
”indication” at 2.5s
“observation” at 99.997% C.L.
G. Bellini et al., PLB 687 (2010) 299-304.
Competition?
In fact it is complementarity!!
S. Abe et al., PRL 100 (2008) 221803.
KamLand: oceanic crust
Borexino: continental crust
Summary of results and perspectives
• Results
– the first clear observation of geoneutrinos at 4.2s ;
– the first measurement of oscillations (reactor antinu) at 1000 km @ 2.9s;
– Georeactor in the Earth core with > 3 TW rejected at 95% C.L.;
• Perspectives:
– Accumulating statistics …. confirmation of BSE/fully radiogenic
Earth??
– Spectroscopy U/Th ratio???
– Future big experiments (LENA, 1000 events/year!!)
– Directionality measurement and Hanohano with 10 kton on the
ocean floor: contribution from the mantle!
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
THANK YOU!!!
Milano
Genova
APC Paris
Perugia
Princeton University
Virginia Tech. University
Dubna JINR Kurchatov
(Russia)
Institute
(Russia)
Munich
(Germany)
Heidelberg
(Germany)
Jagiellonian U.
Cracow
(Poland)
Additional slides
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Future experiments
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
SNO+ at Sudbury, Canada
After SNO: D2O replaced by 1000 tons
of liquid scintillator
M. J. Chen, Earth Moon Planets 99, 221 (2006)
Placed on an old continental crust:
80% of the signal from the crust
(Fiorentini et al., 2005)
BSE: 28-38 events/per year
Mantovani et al., TAUP 2007
April 29th, 2010, GDR neutrino, Paris
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Hanohano at Hawaii
Hawaii Antineutrino Observatory (HANOHANO = "magnificent” in Hawaiian
Project for a 10 kton liquid scintillator
detector, movable and placed on a
deep ocean floor
J. G. Learned et al., XII International Workshop on
Neutrino Telescopes, Venice, 2007.
Since Hawai placed on the U-Th
depleted oceanic crust
70% of the signal from the mantle!
Would lead to very interesting results!
(Fiorentini et al.)
BSE: 60-100 events/per year
Mantovani , TAUP 2007
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LENA at Pyhasalmi, Finland
Project for a 50 kton underground liquid
scintillator detector
K.A. Hochmuth et al. – Astropart. Phys. 27, 2007.
80% of the signal from the continental
crust (Fiorentini et al.)
BSE: 800-1200 events/per year
Scintillator loaded with 0.1% Gd:
- better neutron detection
- moderate directionality information
Mantovani , TAUP 2007
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Directionality of geoneutrinos
•Momentum conservation  neutron starts “moving forwards”
angle (geoneutrino, neutron) < 26o
• directionality degraded during the neutron thermalization
• even a minimal directional information would be sufficient for the
source discrimination
•Reactor & crust antineutrinos horizontal
•Mantle antineutrinos vertical
Gd, Li and B loaded liquid scintillators with which directional
measurement might be possible are under investigation by several
groups
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
First idea about geo-n
Letter from Gamow to Reines, 1953:
Fred Reines and Clyde Cowan
anti-neutrino detector (cca. 1953)
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Types of vulcanism:
mid-ocean ridges
subduction zones (Ands)
island arcs (Japan)
hot spots (Hawaii, Iceland,
Yellowstone)
April 29th, 2010, GDR neutrino, Paris
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Where is concentrated U and Th?
refractory lithophile elements – accumulation in the melt (pegmatites, monazite)
accessories minerals in igneous rocks
(zircon)
Uraninit (oxides of U) + secondary minerals
phosphates, lignit (brown coal)
Heavy grains: accumulation in sandstones;
U: can be dissolved in water!!!! Mobility!!!
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Geoneutrinos: antineutrinos from the Earth
• The main long-lived radioactive elements within the Earth:
238U, 232Th, and 40K
– absolute BSE abundances varies within 10% based on the model;
– ratios of BSE element abundances more stable in different calculations:
• Th/U = 3.9
• K/U = 1.14 x 104
concentration for 238U (Mantovani et al. 2004)
upper continental crust:
2.5 ppm
middle continental crust:
1.6 ppm
lower continental crust:
0.63 ppm
oceanic crust:
0.1 ppm
upper mantle:
6.5 ppb
core
NOTHING
---------------------------------------------------BSE (primordial mantle) 20 ppb
April 29th, 2010, GDR neutrino, Paris
Livia Ludhova for Borexino collaboration
Isotope
T1/2
[ms]
Decay mode
BR
[%]
Qb
[MeV]
8He
119.0
b-n
16
5.3, 7.4
9Li
178.3
b-n
51
1.8, 5.7, 8.6, 10.8, 11.2
Rate: 15.4 eventi/year/100 tons
51 candidates
Accidental coincidences
•Same cuts, just dt instead 20-1280 ms is 2-20 s
0.080±0.001 events/(100ton-year)
13C(a,n)16O
2) Isotopic abundance of 13C: 1.1%
3) 210Po contamination: APo~ 12 cpd/ton
4) Ea=5.3 MeV: Eneutrone ≤ 7.29 MeV for transition to the ground state
MC for 13C (a,n)16O
recoiled proton
12C*
Selection cut
from neutron
16O*
Probability for 210Po nucleus to give (a,n) in pure
In PC it corresponds to (5.0+0.8)10-8
13C
(6.1+0.3) 10-6 (Mc Kee 2008).
(0.014+0.001) events/(100 tons yr)
Muon-induced neutrons
from the rocks
F(En>10 MeV)=7.3×10-10 cm-2s-1
<En> ~ 90 MeV
Borexino shielding:
2m of water
2.5m of PC buffer
lPC(100 MeV) ≅ 70 cm
lPC-ES(100 MeV) ≅ 110 cm
Use neutron spectrum as input for MC simulation:
a) 5×106 events simulated
b) simulated statistics
corresponds to 23 years!
c) 160 events inside Inner Vessel
d) 1 fake anti-n found with 9000p.e.
<0.04 events/(100ton-year) 90% C.L.
Muons crossing the OD
• To remove fast neutrons originated in the Water Tank
we apply a 2ms veto after each detected muon by
the OD
• In correlation with OD tagged muons we have
observed 2 fake anti-n candidates
• The inefficiency of OD muon veto is 5×10-3
• For this background we can set an upper limit of
<0.01 events/(100 ton-year) at 90% C.L.
Predicted
From
reactors
Background
Observed
Probability
to get
N≥Nobs
Geo-n
window
5.0±0.3
0.31±0.05
15
5×10-4
(3.5s)
Reactor-n
window
without
oscillations
16.3±1.1
0.09±0.06
6
Probability
to get
N≤Nobs
5×10-3
(2.9s)